1. Field of the Invention
The present invention relates to a spindle unit having a pressure foot for machining workpieces while pressing down the workpieces to a table by the pressure foot.
2. Description of Related Art
Hitherto, there has been a motor built-in type spindle unit whose machining efficiency is enhanced by increasing a moving speed of a drill, i.e., a machining tool, as disclosed in Japanese Patent Laid-open No. 62-197903 for example. This spindle unit is arranged so as to increase the drill moving speed, i.e., a machining speed, by moving up and down only a rotor shaft i.e., a spindle retaining the drill, in machining workpieces by rotatably and movably supporting the rotor shaft by a housing, i.e., a body portion of the unit. This arrangement allows not only the moving speed to be increased but also vibration occurring in machining to be reduced.
By the way, the prior art spindle unit allows a hole to be made accurately at predetermined position by drilling a workpiece while pressing down a peripheral part of the workpiece to be machined by a pressure foot, i.e., pressure foot means, for pressing the workpiece to a table.
However, the prior art spindle unit having the pressure foot has required other driving means for moving the pressure foot in addition to driving means for moving the rotor shaft so as to be able to move the pressure foot and the rotor shaft separately in machining workpieces. In this case, the driving means of the pressure foot has also played a role of pressing means for pressing the pressure foot to the workpiece and it has been difficult to increase the machining efficiency when compressed air is used for the pressing means because its response is slow in switching the pressing direction.